It would be great if someone could confirm that this indeed would work with Exetel's NBN service (via a HFC connection).

Thanks.

You need to have a modem / router which is a Fibre compatible with WAN port available. In your Billion 7800DXL model, there is no specific WAN port, however, there is a reserved LAN port where you can convert as a WAN port. If the function is working as explained, it should be compatible with HFC.